Metallized organic polymers are utilized in numerous applications requiring conductive or reflective coatings. Metallized films of polyimides (PIm), which have superior thermal stability and dielectric properties, are particularly desirable in the fabrication of flexible printed circuits, and the associated packages and the interconnectors required to assemble microelectronic systems. Conductive connector tapes such as those used in the Tape Automated Bonding (TAB) process described in U.S. Pat. No. 4,472,876 typically have a final electroplated gold layer to facilitate the thermocompressive bonding operation whereby the TAB connector tape is bonded to integrated circuit connector (IC) pads. Thermocompressive bonding processes described in U.S. Pat. No. 4,494,688 require a pure gold layer that is soft and capable of forming strong and durable bonds with the metal comprising the IC pads. Therefore the gold layer must be free of other metals such as arsenic and thallium that can increase its hardness.
A major concern in the electroplating of polyimide substrates for electronic applications is the adhesion of the metal film to the polyimide substrate. The film must stay well-adhered to the substrate not only through the plating and selective etching processes used to form the patterned metal films which comprise the microcircuitry, but also in end-use environments which often include extremes of temperature and humidity. These plating and etching processes take place in highly ionized electrolyte solutions containing ions with high mobilities.
Polyimide substrates containing the pyromellitimide group in the polymer backbone are electroactive. Electroactive (EA) groups, also called redox and charge transfer centers, are described in Haushalter and Krause (Thin Solid Films, 102, 1983, 116-171 "Electroless Metallization of Organic Polymers Using the Polymer as a Redox Reagent: Reaction of Polyimide with Zintl Anions") and disclosed in U.S. Pat. No. 4,775,556 column 5, line 24 through column 7, line 2. In these references the PIm nucleus is reduced electrolessly by using Zintl salts and various chemical reducing agents, and the patent further discloses the utility of such electroactive groups in the electroless deposition of various metallic films. This utility is further disclosed in U.S. Pat. Nos. 4,710,403 and 4,459,330. It is also possible to reduce electroactive nuclei electrolytically in conjunction with electroplating process performed at potentials more negative than the reduction potential of the electroactive nucleus. It has now been recognized that electrolytically reduced EA nuclei can cause problems with the adhesion of electroplated metal films.
U.S. Pat. No. 4,775,556 discloses at column 4, lines 27-40, that "Along with the reduction of the polyimide film is the concomitant diffusion of the counter cation into the film. The size of the counter cation appears to be very important. Alkali metals freely diffuse into the film as reduction proceeds. Intermediate sized quaternary ammonium cations such as tetramethyammonium and tetraethylammonium do diffuse into the polyimide film and reduction of the film to produce a deeply colored radical anion film occurs. However, the ammonium cation appears unstable as the counter cation. This is indicated by a gradual fading of the film color to lighter shades of green."
A commercial ammonium gold cyanide electroplating solution containing arsenic known as Part # 1EHS29 is available from Englehard Corp., Speciality Chemical Division, East Newark, NJ. This solution is specially formulated for use in wire coating applications where the addition of arsenic as a grain refiner to gold electroplating solutions increases process speed and therefore provides an economic advantage over non-arsenic containing electroplating solutions.
This arsenic containing gold electroplating solution is totally unsuitable for use in the present invention because the arsenic would sufficiently harden the gold thus making it unsuitable for thermocompression bonding.
No prior art of which we are aware teaches the utility of the ammonium ion as a charge compensating counter cation capable of preventing the electrochemical reduction of electroactive substrates in electroplating processes conducted at potentials more negative than the reduction potential of such substrates.